Many types of input devices are presently available for performing operations in a computing system, such as buttons or keys, mice, trackballs, touch panels, joysticks, touch screens and the like. Touch screens, in particular, are becoming increasingly popular because of their ease and versatility of operation as well as their declining price. Touch screens can include a touch panel, which can be a clear panel with a touch-sensitive surface. The touch panel can be positioned in front of a display screen so that the touch-sensitive surface covers the viewable area of the display screen. Touch screens can allow a user to make selections and move a cursor by simply touching the display screen via a finger or stylus. In general, the touch screen can recognize the touch and position of the touch on the display screen, and the computing system can interpret the touch and thereafter perform an action based on the touch event.
Touch panels can include an array of touch sensors capable of detecting touch events (the touching of fingers upon a touch-sensitive surface). Some current touch panels are able to detect multiple touches (the touching of fingers upon a touch-sensitive surface at distinct locations at about the same time) and near touches (fingers within the near-field detection capabilities of their touch sensors), and identify and track their locations. Examples of multi-touch sensor panels are described in Applicant's U.S. application Ser. No. 10/842,862 entitled “Multipoint Touchscreen,” filed on May 6, 2004 and published as U.S. Published Application No. 2006/0097991 on May 11, 2006, the contents of which are incorporated by reference herein.
Capacitive touch sensor panels can be formed as an array of rows and columns of sensors on opposing sides of a touch substrate. For example, the rows can form drive electrodes on one surface of the touch substrate and the columns can form sense electrodes on the opposing surface. To scan a sensor panel, a stimulus can be applied to one row with all other rows held at DC voltage levels. When a row is stimulated, a modulated output signal can appear on the columns of the sensor panel. The columns can be connected to analog channels (also referred to herein as event detection and demodulation circuits). For every row that is stimulated, each analog channel connected to a column generates an output value representative of an amount of change in the modulated output signal due to a touch event occurring at the sensor located at the intersection of the stimulated row and the connected column. After analog channel output values are obtained for every column in the sensor panel, a new row is stimulated (with all other rows once again held at DC voltage levels), and additional analog channel output values are obtained. When all rows have been stimulated and analog channel output values have been obtained, the sensor panel is said to have been “scanned,” and a complete “image” of touch can be obtained over the entire sensor panel. This image of touch can include an analog channel output value for every pixel (row and column) in the panel, each output value representative of the amount of touch that was detected at that particular location.
The manufacturing cost of a two-surface sensor panel as described above is generally higher than if only one surface of the substrate was needed for sensor circuitry. In addition, when the touch substrate overlays a display device (e.g., an LCD), circuitry on the second surface typically increases light loss as compared to a single surface system. Therefore, it is desirable for cost and performance reasons to realize a multi-touch sensor that only needs one surface of the touch substrate for sensor circuitry.